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Subject[PATCH 13/13] jump_label,x86: Allow short NOPs
Now that objtool is able to rewrite jump_label instructions, have the
compiler emit a JMP, such that it can decide on the optimal encoding,
and set jump_entry::key bit1 to indicate that objtool should rewrite
the instruction to a matching NOP.

For x86_64-allyesconfig this gives:

jl\ NOP JMP
short: 22997 124
long: 30874 90

IOW, we save (22997+124) * 3 bytes of kernel text in hotpaths.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/include/asm/jump_label.h | 18 ++++++++++++++++++
1 file changed, 18 insertions(+)

--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -28,6 +28,22 @@
_ASM_PTR "%c0 + %c1 - .\n\t" \
".popsection \n\t"

+#ifdef CONFIG_STACK_VALIDATION
+
+static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
+{
+ asm_volatile_goto("1:"
+ "jmp %l[l_yes] # objtool NOPs this \n\t"
+ JUMP_TABLE_ENTRY
+ : : "i" (key), "i" (2 | branch) : : l_yes);
+
+ return false;
+l_yes:
+ return true;
+}
+
+#else
+
static __always_inline bool arch_static_branch(struct static_key * const key, const bool branch)
{
asm_volatile_goto("1:"
@@ -40,6 +56,8 @@ static __always_inline bool arch_static_
return true;
}

+#endif /* STACK_VALIDATION */
+
static __always_inline bool arch_static_branch_jump(struct static_key * const key, const bool branch)
{
asm_volatile_goto("1:"

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